I am starting to use my Samsung YP-P3 and exploring the best way to support it in MTP mode and also with Linux. I won't make a very late review of this popular player, but just tell the results of my first tests with it.

The P3 is perfectly recognized on Linux (in both MTP and MSC modes) and is much faster than my Sony E430. MTP queries are performed in a blink of an eye. Of course the device is empty for the moment, but the little Sony always "think" longer before it sends back its answers. I didn't meter but transfer speed is much higher too. You can clearly feel it is a more powerful premium device compared to the entry level Walkman that was my previous only MTP test device.

I understand now why people face problems copying mp4 video and Flac audio files to the Samsung P3. Here is the supported formats list the P3 exhibits in MTP mode (as recognized by libmtp):

These 2 types should tell the PC software that the device supports the mp4 container (.mp4 files) and the flac audio file format. That explains why people could complain that they can't copy mp4 and flac to their P3 while in MTP mode.

Maybe Samsung engineers thought that Apple Quicktime container format was right to declare mp4 format support. I've read on the web that this extension list: [mov, qt, mp4, m4v, m4a, m4p, m4b, m4r, k3g, skm, 3gp, 3g2] can be linked to Quicktime.
Still they forgot a Flac flag. I know that a flac stream can be put inside an ogg container. I will try this on the P3.

Now can we suppose that Undefined audio file could have been a fall-back flag for flac files as well any other audio format? In that case your MTP software knowing Flac is audio could still send it to the P3. The same could be said about Quicktime (Qt). But the P3 does not seem to know .mov or .qt files which are the most famous Qt extension. So why did they make this choice?
A good media software must take peculiarities in account or let you add your file types in order to fully support the Samsung P3 in MTP mode.

[Edit december, 8th]
After more investigations, I've understood that the ogg support claimed by the P3 comes from a tweak inside the libmtp driver code. You can modify the way the library handle the device to help solving some issues. For example the Ogg support I see was added via a special flag, because someone has checked it works. So I tried to enable Flac support declaration using the dedicated flag too. Then the P3 claims it loves Flac files also. But like many people have experienced it, the P3 doesn't react well when Flac files are transmitted via MTP. The database entry remain incomplete (even if I try to edit it afterward) and the player shows a track with unknown properties even if the cover shows when you play the file. Conclusion: with Flac go MSC.
Strangely for AAC audio files it's quite the opposite, they are more suited for the P3 MTP mode. First you must use .aac or m4a extensions. An audio only .mp4 file will be rejected by the P3.
Copied in MSC mode the file will be playable but will be full unknown content, but this time metadatas for AAC files are welcomed in MTP mode. The embedded cover is not recognized but you can send the cover art with a MTP album linked to the files. So if you are into AAC, go MTP.